Remove devices from Microsoft 365 Defender portal

The devices will remain listed until the retention period expires (30 days to 180 days), as is listed in the documentation.
https://docs.microsoft.com/en-us/microsoft-365/security/defender-endpoint/offboard-machines?view=o365-worldwide

I do agree it would be more straight forward if the devices were also removed from the device list, but it does not appear to be possible right now.